Electric heating units and methods of making the same



Oct. 1, 1957 c. H. YOHE 2,308,492

' ELECTRIC HEATING UNITS AND METHODS OF MAKING THE SAME Fileh July 26, 1954 2 Sheets-Sheet 1 1-762 Supply of Storage Ra/l Res/sfance 50 Conducfor v 43 42 0f f/p/shed ,S'u, 0p/y of i Braiding or I heafing Un/f 30 f/lzzzlzenfs Wm mg Head IN V EN TOR. Charles H Yohe BY p y Melzz/ Siva/r ELECTRIC HEATING UNITS AND METHODS OF MAKING THE SAME Filed July 26, 1954 C. H. YOHE Oct. 1, 1957 2 Sheets-Sheet 2 mm w vb N% Q ,Q QRw N\ 6R M m? 0 {Q 0Q 0 9% Q G United States Patent ELECTRIC HEATING UNITS AND METHODS OF MAKJNG THE SAME Charles H. Yohe, Glen Ellyn, 111., assignor to General Electric Company, a corporation of New York Application July 26, 1954, Serial No. 445,659

Claims. (Cl. 201-67) The present invention relates to electric heating units and methods of making the same; and it is the general object of the invention to provide an electric heater of the sheathed resistance conductor type that is of exceedingly simple construction and arrangement and that lends itself to ready and economical manufacture.

Another object of the invention is to provide an electric heater of the type noted that may be readily manufactured incorporating a sheath of exceedingly small diameter and of exceedingly great length.

Another object of the invention is to provide an electric heating unit of the type noted, wherein the cross-sectional area of the resistance conductor imbedded in the body of electrical-insulating and heat conducting material and enclosed by the sheath is at least 10% of the crosssectional area of the interposed body.

Another object of the invention is to provide an electric heating unit of the character noted, wherein the body of material disposed between the metallic resistance conductor and the enclosing metallic sheath consists essentially of siliceous filaments retained in compression between the resistance conductor and the sheath to provide a highly compacted resilient body having good electrical insulating and thermal conducting properties.

A further object of the invention is to provide an electric heating unit of the character described, wherein the siliceous filaments mentioned consist essentially of glass from which a preponderance of the glass-forming metaloxide constituents have been leached.

A further object of the invention is to provide an improved and exceedingly simple method of making an electric heating unit of the character described.

A still further object of the invention is to provide an improved method of making such electric heating units upon a continuous basis so that such heating units may be manufactured in exceedingly great lengths.

Further features of the invention pertain to the particular arrangement of the elements of the electric heating unit and of the steps of the method of making the same, whereby the above-outlined and additional operating features thereof are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification, taken in connection with the accompanying drawings, in which:

Figure l is a fragmentary side view of an electric heating unit of the sheathed resistance conductor type embodying the present invention;

Fig. 2 is an enlarged sectional view of the heating unit, taken in the direction of the arrows along the line 2-2 in Fig. 1;

Figs. 3 to 9, inclusive, are enlarged sectional views, similar to Fig. 2, and illustrating corresponding modifications of the heating unit of Figs. 1 and 2;

Fig. 10 is a schematic diagram of apparatus suitable for use in making the heating unit of Figs. 1 and 2, and also 2,808,492 Patented Oct. 1, 1957 ICC illustrating the steps of the method of the present invention;

Fig. 11 is an enlarged schematic diagram of the arrangement of the sets of forming rolls incorporated in the apparatus of Fig. 10;

Figs. 12 to 16, inclusive, are further enlarged fragmentary progressive sectional views of the heating unit assembly and the respective sets of forming rolls incorporated in the apparatus of Fig. 11, illustrating the progressive forming steps involved in the method, and taken in the direction of the arrows along the respective lines: l212, 1313, 14-l4, 15-15 and 16-16 in Fig. 11;

Fi 17 is an enlarged schematic diagram of the arrangement of the sets of compressing rolls incorporated in the apparatus of Fig. 10; and

Figs. 18 to 23, inclusive, are further enlarged progressive sectional views of the heating unit assembly, illustrating the progressive compressing steps involved in the method, and taken in the direction of the arrows along the respective lines: 18-l3, 19-19, 2-2tl, 2l-21, 22-22 and 23-23 in Fig. 17.

Referring now to Figs. 1 and 2 of the drawings, the electric heating unit 34) there illustrated, and embodying the features of the present invention, is of the sheathed resistance conductor type, and comprises a metallic resistance conductor 31, a layer 32 of fibrous silica or siliceous material enclosing the resistance conductor 31, and a tubular metallic sheath 33 enclosing both the layer 32 and the resistance conductor 31 and retaining the layer 32 in compression about the resistance conductor 31 so that the resistance conductor 31 is maintained in spaced relation with respect to the sheath 33 and substantially centrally therein. in the finished heating unit 30, the resistance conductor 31 may project from the ends of the sheath 33 to provide appropriate electrical terminals 31a, as indicated on the right-hand side in Fig. l; and the ends of the sheath 3?- may be closed with porcelain or glass plugs, as indicated at 34, in order to seal the interior of the sheath 33 against the entrance of moisture thereinto. The plugs or seals 34 may, for example, be of the construction and arrangement disclosed in U. S. Patent No. 1,992,787, granted on February 26, 1935, to Robert I. Sutton.

In the arrangement, the resistance conductor 31 may be formed of any suitable resistance conductor material, but is usually formed of a suitable nickelchromium alloy; and likewise, the sheath 33 may be formed of any suitable sheath material, and is frequently formed of copper, brass, aluminum or a suitable nickel-chromiumiron alloy.

The layer 32 of siliceous fibers may take Wide variety of forms, but it is essentially textile in character, and preferably comprises a braided covering or jacket of siliceous filament in yarn or thread form. Alternatively, the layer 32 may comprise a wrapping of textile tape formed of siliceous yarn or thread. In any case, the layer 32 of siliceous filaments is arranged in enclosing relation With respect to the resistance conductor 31 and is supported thereby to prevent displacement thereof incident to enclosing both the layer 32 and the resistance conductor 31 in the sheath 33 and the subsequent working of the sheath 33, as explained more fully hereinafter, to compact the resilient layer 32 of siliceous filaments about the resistance conductor 31 and firmly in place in the space between the resistance conductor 31 and the sheath 33, whereupon the layer 32 of siliceous filament becomes a highly compacted but nevertheless resilient body having exceptional qualities of electrical insulation and thermal conductivity.

The siliceous filaments 0f the layer 32 essentially comprise glass fiber from Which a preponderance of the glassaeoaaea forming metal-oxide constituents have been leached; and the product Refrasil manufactured by The H. I. Thompson Company is quite satisfactory. This product is formed by leaching type E glass fiber with a mineral acid (preferably HCl or HNOs) for several hours at an elevated temperature, which product has a softening point of about 2350 F., and a melting point somewhat in excess of 3000 F.; and a typical composition of this product is as follows:

The 2.32%, unaccounted for, includes some water of hydration, some water of absorption, some uncombusted carbonaceous residue, some undetermined impurities present in the original glass mix, and minor analytical errors.

A suitable type E glass fiber that may be leached is the product ECC-ll Fiberglas manufactured by the Owens-Corning Fiber Glass Corporation. This product formed of type E glass has a softening point of about -ll00 F. and a melting point of about 1400 F. and a typical composition of this product is as follows:

Percent Silica (SiOz) 53.76 Aluminum oxide (A1203) 15.29 Titanium oxide (TiOz) 0.08 Iron oxide (F6203) 0.23

Calcium oxide (CaO) 16.80

Magnesium oxide (MgO) 5.10 Alakli as sodium oxide (NazO) 0.42 Boric anhydride (BzOa) 6.60

The 1.72%, unaccounted for, includes some uncombusted carbonaceous residue, some undetermined impurities present in the original glass mix, and minor analytical errors.

More particularly, the leached glass product specified may be produced from the original glass product specified by leaching the original product with HCl of about 11.2% initial concentration at a temperature of about 140 F., for about five hours. Thereafter, the acid-leached product is washed acid-free, air-dried, and then fired at about 1700 F., for about eight hours. It will be appreciated that in the leaching step a preponderance of the glassforming metal-oxide constituents are extracted, so that in the final product the ratio of the silica to the small remaining glass-forming metal-oxide constituents is about 99%; whereby the final product is substantially pure silica. The details of the method generally described above for leaching the original product specified in order to produce the final product specified are disclosed in U. S. Patent No. 2,491,761, granted on December 20, 1949, to Leon Parker and Alexander Cole.

Alternatively, the siliceous filaments of the resilient layer 32 may comprise the product Fiberfrax manufactured by the Carborundum Company, or the product RF800 manufactured by the Johns-Manville Company; however, the Refrasil product initially specified is exceedingly satisfactory, as it comprises substantially 99% pure silica and possesses the exceedingly high softening and melting points previously noted.

In making the heating unit 30 of a normal length (several feet), it is entirely satisfactory merely to apply the layer 32 of the siliceous filaments to the resistance conductor 31, and then place this subassemblyinto the preformed tubular sheath 33. Thereafter, the composite assembly may be subjected to a cold-working step (cold rolling or cold swaging) of the general character of that disclosed in U. S. Patent No. 2,677,172, granted on May 4, 1954, to Sterling A. Oakley; whereby the cross-sectional area of the sheath 33 is substantially reduced, effecting a corresponding compression of the layer 32 about the resistance conductor 31 and firm packing thereof in place between the resistance conductor 3i and the sheath 33, so that the resistance conductor Si is retained securely in place substantially centrally of the sheath 33. After the composite assembly is thus cold-worked, the ends of the sheath 33 may be stripped back, or otherwise removed, in order to accommodate the formation of the terminals 31a on the ends of the resistance conductor 31 and the closing of the ends of the sheath 33 with the porcelain or glass plugs 34. In this connection, it is noted that ordinarily in the manufacture of the heating units 30 separate enlarged steel terminals, not shown, are suitably secured to the exposed ends of the resistance conductor 31 and embedded in the porcelain or glass plugs 34 provided in the ends of the sheath 33.

In making the heating unit 30 of an exceedingly long length, a continuous method may be employed that comprises the apparatus 40 diagrammatically shown in Fig. 10; which apparatus 40 includes a supply roll 41 from which the resistance conductor 3.1 is supplied, a braiding or Wrapping head 42 to which the siliceous filaments are supplied in the form of threads or tapes, as indicated at 43, from the source indicated at 44, a supply roll 45 from which the flat stock 33a of the sheath 33 is supplied, a set of forming rolls 47, an electric seam welder 48, a set of compressing rolls 43", and a storage roll 50 to which the finished heating unit 30 is supplied in a continuous length. More particularly, the resistance conductor 31 is continuously fed from the supply roll 4?. to the head 42, whereupon. the siliceous filamentary material 43 is continuously applied thereto from the source 44- in order to produce, as a covering or jacket, the textile layer 32 supported by the resistance conductor 31. The resistance conductor 31 supporting the layer 32 of siliceous material is then fed continuously from the head 42 over a guide roller 51 into the set of forming rolls 47. The fiat stock 33a of the sheath 33 may be initially of ribbon-like character and is continuously fed from the supply roll 45 over a guide roller 52 into the set of forming rolls 47. In the set of forming rolls 47 the ribbonlike stock 33a is continuously formed into the sheath 33 about the resilient layer 32 of siliceous filaments; and therefrom the assembly is continuously fed into the electric seam welder wherein the adjacent longitudinal edges of the stock 33a are welded together to form the tubular sheath 33. From the electric seam welder 48, the assembly is continuously fed into the compressing rolls 49, wherein the assembly is cold-worked to produce the finished heating unit 30; and therefrom the finished heating unit 30 is continuously fcd over a guide roller 53 and thence onto the storage roll 50.

The details of the method steps involving the set of forming rolls 47 may best be understood by reference to Figs. 11 to 16, inclusive, wherein the longitudinal array of the individual forming rolls is diagrammatically illustrated. As shown in Fig. 11, five individual pairs of forming rolls 6162, 636i, asap, 6768 and 6970 are arranged in tandem longitudinal relation in the order named. The flat ribbon-like stock 33a is sup plied between the first pair of forming rolls 61-62 and passes progressively through the remaining pairs of forming rolls 6364, 65-66, etc. As indicated in Figs. l2, l3 and 14, the flat ribbon-like stock 33:! is progressively cold-worked toward a hollow transverse configuration; and between the third set of forming rolls 6S66 and the fourth set of forming rolls 676, the subassembly of the resistance conductor 31 and the textile covering or jacket 32 of the siliceous filaments is fed over a guide roller 71 into the thus formed stock 33a. In the fourth set of forming rolls 6768, the stock 33a is further formed toward a tubular configuration enclosing the subassembly of the resistance conductor 31 and the textile covering or jacket 32; and in the fifth set of forming rolls 6970, the stock 33a is formed completely into its tubular configuration to provide the sheath 33, the adjacent edges of the stock 33a being disposed in engagement with each other to provide the longitudinal scam in the sheath 33.

In the electric seam welder 48, the adjacent engaging edges of the stock 33a are welded together to provide the finished longitudinal seam in the sheath 33.

The details of the method steps involving the set of compressing rolls 49 may best be understood by reference to Figs. 17 to 23, inclusive, wherein the longitudinal array of the individual compressing rolls is diagrammatically illustrated. As shown in Fig. 17, five individual pairs of compressing rolls 81-82, 83-84, 85-36, ST-88 and 8990 are arranged in tandem longitudinal relation in the order named. The heating unit 30 passes through the successive pairs of compressing rolls 81-82, 33-34, etc., whereby the cross-sectional area. thereof is gradually reduced; and in the arrangement the pairs of compressing rolls 81-452, t3384, 35S6 and $788 comprise elliptical passes that are displaced substantially 90 angles with respect to each other so that the corresponding elliptical sections produced thereby are orient ed in the general manner respectively indicated in Figs. 19, 20, 21 and 22. Accordingly, the diameter of the sheath 33 entering the first set of compressing rolls 81-82, as shown in Fig. 18, is considerably larger than the diameter of the sheath 33 leaving the fifth set of compressing rolls 39 96, as shown in Fig. 23. The orientation of the several elliptical passes, as described above, is advantageous in preventing finning of the sheath 33 as the diameter there-of is progressively reduced and as the resilient layer 32 of siliceous filaments 32 is compressed about the resistance conductor 31 and firmly in place between the resistance conductor 31 and the sheath 33.

In passing, it is noted that in the event it is necessary to apply a lubricant, wax, or the like, to the yarn or thread to insure proper operation of the braiding or wrapping head 42, the applied lubricant, etc., should be removed from the subassembly before the sheath 33 is applied thereto. For example, the applied lubricant, etc, may be simply burned from the subassernbly by passing the same through an appropriate oven following the head 42 and preceding the guide roller 51. This is noted as it is highly desirable that no foreign materials, moisture, etc., be contained within the sheath 33 of the finished heating unit 30.

By way of a constructional example, it is noted that the tubular sheath 33, as illustrated in 18, may have an initially outside diameter of approximately 0.160, and the tubular sheath 33, as illustrated in Fig. 23, may have a finished outside diameter of approximately 0.125. The finished sheath 33 may have a finished inside diameter of approximately 0.080 and the resistance conductor 31 may have an outside diameter of approximately 0.030; whereby the cross-sectional area of the resistance conductor 31 is somewhat in excess of 16% of the crosssectional area of the imbedding resilient layer 32 of siliceous filaments. The final density of the resilient layer 32 may be of the order of 1.2 to 1.4 grams per cubic centimeter.

The above-described continuous method of manufacturing the heating unit 30 is very advantageous, as there is no critical limitation upon the ultimate length thereof and there is excellent centralization of the resistance conductor 31 and a substantially uniform insulation thickness between the resistance conductor 31 and the metallic sheath 33.

Finally, it is pointed out that the heating unit may take a wide variety of forms with respect to the number of resistance conductors enclosed in the sheath thereof, with respect to the configuration of the sheath, etc., as shown in Figs. 3 to 9, inclusive. In each case, it is contemplated that each resistance conductor is separately enclosed in its own resilient layer of siliceous filaments, and then the several resistance conductors and enclosing layers of siliceous filaments are mutually enclosed by the associated sheath. Thereafter, the sheath is cold-worked, in the general manner previously explained, in order to reduce the cross-sectional area thereof so as to compress the resilient layers of siliceous filaments about the respective resistance conductors and into firm engagement therewith and with the enclosing sheath; whereby a composite bundle is formed of the resistance conductors and the respective enclosing resilent layers of siliceous filaments, filling the space within the enclosing sheath and retaining the respective resistance conductors in mutually spacedapart relation.

As shown in Fig. 3, the heating unit comprises three individual metallic resistance conductors 101 substantially circular in cross-section, three respective individual resilient layers of siliceous filaments 102, and a common enclosing metallic sheath 103 substantially circular in cross-section.

As shown in Fig. 4, the heating unit comprises seven individual metallic resistance conductors 111 substantially circular in cross-section, seven respective individual resilient layers of siliceous filaments 112, and a common enclosing metallic sheath 113 substantially circular in cross-section.

As shown in Fig. 5, the heating unit comprises a single metallic resistance conductor 121 substantially elongated oval in cross-section, a single individual resilient layer of siliceous filaments 122, and an enclosing metallic sheath 123 substantially elongated oval in crosssection.

As shown in Fig. 6, the heating unit comprises three individual metallic resistance conductors 131 substantially circular in cross-section, three respective individual resilient layers of siliceous filaments 132, and a common enclosing metallic sheath 133 substantially triangular in cross-section.

As shown in Fig. 7, the heating unit comprises three individual metallic resistance conductors 141a, 1411; and 141a, three respective individual resilient layers of siliceous filaments 142, and a common enclosing metallic sheath 143 substantially circular in cross-section. In the heating unit 140, the resistance conductors 14102 are substantially circular in cross-section and disposed on opposite sides of the centrally arranged resistance conductor 1411) that is substantially elongated oval in cross-section, the resistance conductors 141a being somewhat smaller in crosssectional area than the centrally disposed resistance conductor 14112. This arrangement permits of considerable flexibility with reference to the control of the heating unit 140 in service, since it is apparent that the resistance conductors 141a and 141b possess somewhat different current-carrying capacities.

As shown in Fig. 8, the heating unit comprises three individual metallic resistance conductors 151a, 151k and 151a, three respective individual resilient layers of siliceous filaments 152, and a common enclosing metallic sheath 153 substantially square in cross-section. in the heating unit 150, the resistance conductors 151a are sub stantially square in cross-section and disposed on one side of the resistance conductor 151k that is substantially rectangular in cross-section, the resistance conductors 151a being substantially smaller in cross-sectional area than the resistance conductor 151E). This arrangement permits of considerable flexibility with reference to the control of the heating unit 150 in service, since it is apparent that the resistance conductors 151a and 151b possess considerably different current-carrying capacities.

As shown in Fig. 9, the heating unit comprises two individual metallic resistance conductors 161 substan- 7 tially circular in cross-section, two respective individual resilient layers of siliceous filaments 162, and a common enclosing metallic sheath 163. In the heating unit 160, the sheath 163 is of modified elongated oval cross-section, the opposed central portions 163a of the elongated sides thereof being pinched together so as mechanically to isolate the two resistance conductors 161 in two laterally spaced-apart compartments thus formed in the sheath 163.

In view of the foregoing, it is apparent that there has been provided an electric heating unit of improved and simplified construction and arrangement, together with a method of making the same of improved and simplified arrangement.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An electric heating unit comprising a metallic resistance conductor, a layer of filaments enclosing said resistance conductor, said filaments consisting essentially of silica, also containing non-siliceous metal oxides, the ratio of the silica to the metal oxides being in excess of 9, said filaments having a melting point in excess of 1950 F., and a metallic sheath enclosing both said layer of filaments and said resistance conductor and retaining said layer of filaments in compression about said resistance conductor so that said resistance conductor is maintained in spaced relation with respect to said sheath.

2. The electric heating unit set forth in claim 1, wherein said filaments are formed of glass from which a preponderance of the glass-forming metal-oxide constituents have been leached.

3. The electric heating unit set forth in claim 1, wherein said layer of filaments is essentially in textile form.

4. The electric heating unit set forth in claim 1, wherein said layer of filaments is essentially in the form of a braided casing.

5. The electric heating unit set forth in claim 1, wherein said layer of filaments is essentially in the form of a wrapping of textile tape.

6. An electric heating unit comprising a metallic resistance conductor, a layer of elongated filaments enclosing said resistance conductor, said filaments containing more than 90% silica and less than 10% glass-forming metal-oxides and having a melting point in excess of 1950 F, and a metallic sheath enclosing both said layer of filaments and said resistance conductor and retaining said layer of filaments in compression about said resistance conductor so that said resistance conductor is maintained in spaced relation with respect to said sheath.

7. An electric heating unit comprising a plurality of metallic resistance conductors arranged in mutually spaced-apart relation, a plurality of layers of filaments respectively enclosing said resistance conductors, said filaments consisting essentially of silica, also containing non-siliceous metal oxides, the ratio of the silica to the metal oxides being in excess of 9, said filaments having a melting point in excess of 1950 F., and a metallic sheath commonly enclosing said layers of filament and said resistance conductors and retaining said layers of filaments in compression about the respective ones of said resistance conductors and also retaining said layer of filaments and said resistance conductors in a compressed bundle so that said resistance conductors are maintained in mutually spaced-apart relation with respect to each other and also in spaced relation with respect to said sheath.

8. The method of making an electric heating unit of the sheathed resistance conductor type comprising pro- (7 QJI viding an elongated metallic resistance conductor, enclosing said resistance conductor in an elongated resilient layer of filaments consisting essentially of silica, also containing non-siliceous metal oxides, the ratio of the silica to the metal oxides being in excess of 9, providing an elongated strip of metal stock, forming said strip into an elongated sheath enclosing both said layer of filaments and said resistance conductor and provided with an elongated junction between the adjacent edges of said strip in said sheath, Welding the adjacent edges of said strip in said sheath to provide an elongated seam in said sheath, and then working said sheath along the length thereof to reduce the cross-sectional area of said sheath sufficiently to compress said resilient layer of filaments into a dense fibrous heat-conducting and electrical-insulat ing mass disposed about said resistance conductor and firmly in place between said resistance conductor and said sheath, said mass being characterized by high thermal stability retaining its fibrous form and its heat-conducting and electrical-insulating properties over an operating temperature range extending above 1950" F.

9. The continuous method of making an electric heating unit of the sheathed resistance conductor type comprising providing an elongated metallic resistance conductor, continuously feeding said resistance conductor past four tandem related stations, enclosing said resistance conductor as it passes the first of said stations in a resilient layer of filaments consisting essentially of silica, also containing non-siliceous metal oxides, the ratio of the silica to the metal oxides being in excess of 9, providing an elongated strip of metal stock, continuously feeding said strip past the second and the succeeding ones of said stations, forming said strip as the assembly passes the second of said stations into a sheath enclosing both said layer of filaments and said resistance conductor and provided with a junction between the adjacent edges of said strip in said sheath welding the adjacent edges of said strip in said sheath as the assembly passes the third of said stations to close the seam in said sheath, and working said sheath as the assembly passes the fourth of said stations to reduce the cross-sectional area thereof sufliciently to compress said resilient layer of filaments into a dense fibrous heat-conducting and electrical-insulating mass disposed about said resistance conductor and firmly in place between said resistance conductor and said sheath, said mass being characterized by high thermal stability retaining its fibrous form and its heat-conducting and electrical-insulating properties over an operating temperature range extending above 1950 F.

10. An electric heating unit comprising a metallic resistance conductor, a dense mass of filaments enclosing said resistance conductor, said filaments consisting essentially of silica, also containing non-siliceous metal oxides, the ratio of the silica to the metal oxides being in excess of 9, and a metallic sheath enclosing both said mass of filaments and said resistance conductor and retaining said mass of filaments in compression therein and about said resistance conductor so as to provide a heatconducting and electrical-insulating layer between said resistance conductor and said sheath, said mass of filaments being characterized by high thermal stability retaining its filamentary form and its heat-conducting and electrical-insulating properties over an operating temperature range extending above 1950 F.

References Cited in the file of this patent UNITED STATES PATENTS 1,359,400 Lightfoot Nov. 16, 1920 2,186,442 Youmans Jan. 9, 1940 2,251,697 Van Daam et al. Aug. 5, 194-1 2,419,655 Reiser Apr. 29, 1947 

1. AN ELECTRIC HEATING UNIT COMPRISING A METALLIC RESISTANCE CONDUCTOR, A LAYER OF FILAMENTS ENCLOSING SAID RESISTANCE CONDUCTOR, SAID FILAMENTS CONSISTING ESSENTIALLY OF SILICA, ALSO CONTAINING NON-SILICOUS METAL OXIDES, THE RATIO OF THE SILICA TO THE METHAL OXIDES BEING IN EXCESS OF 9, SAID FILAMENTS HAVING A MELTING POINT IN EXCESS OF 1950*F., AND A METALLIC SHEATH ENCLOSING BOTH SAID LAYER OF FLAMENTS AND SAID RESISTANCE CONDUCTOR AND RETAINING SAID LAYER OF FILAMENTS IN COMPRESSION ABOUT SAID RESISTANCE CONDUCTOR SO THAT SAID RESISTANCE CONDUCTOR IS MAINTAINED IN SPACED RELATIN WITH RESPECT TO SAID SHEATH. 